Field of the Invention
The present invention relates to cylindrical magnet systems as used in imaging systems such as MRI (Magnetic Resonance Imaging) systems.
Description of the Prior Art
FIG. 1 shows a radial cross-section through a typical magnet system for use in an imaging system. A cylindrical magnet 10, typically comprising superconducting coils mounted on a former or other mechanical support structure, is positioned within a cryostat, having a cryogen vessel 12, thermal radiation shield 16 and outer vacuum container (OVC) 14. The cryogen vessel 12 contains a quantity of liquid cryogen 15, for example helium, which holds the superconducting magnet at a temperature below its transition temperature.
The magnet is essentially rotationally symmetrical about axis A-A. The term “axial” is used in the present document to indicate a direction parallel to axis A-A, while the term “radial” means a direction perpendicular to axis A-A and passing through that axis. The cryogen vessel 12 is itself cylindrical, having an outer cylindrical wall 12a, an inner cylindrical bore tube 12b, and substantially planar annular end caps (not visible in FIG. 1). An outer vacuum container (OVC) 14 surrounds the cryogen vessel. It also is itself cylindrical, having an outer cylindrical wall 14a, an inner cylindrical bore tube 14b, and substantially planar annular end caps (not visible in FIG. 1). A hard vacuum is provided in the volume between the OVC 12 and the cryogen vessel 14, providing effective thermal insulation. A thermal radiation shield 16 is placed in the evacuated volume. This is typically not a fully closed vessel, but is essentially cylindrical, having an outer cylindrical wall 16a, an inner cylindrical bore tube 16b, and substantially planar annular end caps (not visible in FIG. 1). The thermal radiation shield 16 serves to intercept radiated heat from the OVC 14 before it reaches the cryogen vessel 12. The thermal radiation shield 16 is cooled, for example by an active cryogenic refrigerator 17, or by escaping cryogen vapor.
In alternative arrangements, the magnet is not housed within a cryogen vessel, but is cooled in some other way: either by a low cryogen inventory arrangement such as a cooling loop, or a ‘dry’ arrangement in which a cryogenic refrigerator is thermally linked to the magnet by solid thermal conduction. In such arrangements, there is no cryogen reservoir to absorb heat generated by ohmic heating of various conductive components by eddy currents.
The OVC bore tube 14b must be mechanically strong and vacuum tight, to withstand vacuum loading both radially and axially. Conventionally, it made of stainless steel. The cryogen vessel bore tube 12b, if any, must be strong and capable of withstanding the pressure of cryogen gas within the cryogen vessel. Typically, this is also of stainless steel. The bore tube 16b of the thermal radiation shield 16 must be impervious to infra-red radiation. It is preferably lightweight. It is typically made of aluminum.
The present invention may be applied in all such cases.
In order to provide an imaging capability, a set of gradient coils 20 are provided within the bore of the superconducting magnet. These are usually arranged as a hollow cylindrical, resin-impregnated block, containing coils which generate orthogonal oscillating magnetic field gradients in three dimensions.
During an imaging procedure, the gradient coils 20 generate rapidly oscillating magnetic fields, for example at a frequency of about 1500 Hz-2500 Hz. Stray fields from the gradient coils generate eddy currents in the closest conductive surface, typically a bore tube 14b, of the OVC. As described below, this can in turn lead to induced eddy currents on other conductive surfaces, such as metal parts of the cryostat, in particular in metal bore tubes 16b, 12b of thermal shield and cryogen vessel, and also in the structure of the magnet 10. The eddy currents produced in the material of the OVC 14 will help to shield the thermal radiation shield 16 and cryogenically cooled components such as cryogen vessel bore tube 12b, magnet coils and magnet former 10 from stray fields from the gradient coils 20. However, because of the constant background magnetic field produced by the magnet, those eddy currents produce Lorentz forces, resulting in mechanical vibrations in the bore tube of the OVC. Further mechanical vibrations result from mechanical vibration of the gradient coil assembly itself, caused by Lorentz forces acting on the conductors of the gradient coil assembly 20 which carry significant alternating currents. Mechanical vibration of the gradient coil assembly also causes noise by direct vibration of air within the bore.
These bore-tube mechanical vibrations, in the constant background magnetic field of the magnet 10, will in turn induce secondary eddy currents in conductive materials, such as the bore tube 16b of the thermal radiation shield. The secondary eddy currents will of course generate magnetic fields, known as secondary magnetic fields. These may interfere with imaging, and produce mechanical vibrations and secondary stray fields in that region. The secondary stray fields also induce tertiary eddy currents in nearby conductive surfaces. These tertiary eddy currents will, in turn, generate tertiary magnetic fields, and so on. By this mechanism, a substantial amount of energy may be transferred from the gradient coil to the cold structure of the magnet, despite several layers of shielding. This can result in significant cryogen loss in conventional magnets, and quenching in types of magnets with little or no cryogen inventory, such as the so-called “dry” magnets discussed above.
The bore tube 16b of the thermal radiation shield 16 is preferably thermally and electrically conductive to provide electromagnetic shielding of the magnet from the gradient coils in addition to its function of providing a cold surface and blocking infrared radiation from the OVC 14 the cryogen vessel 12 or the magnet structure 10.
A particular difficulty arises when, as is typical, the frequency of oscillation of the gradient magnetic fields is close to the resonant frequency of the bore tubes 12b, 14b, 16b. It is known that a number of concentric tubes of similar diameters, such as the bore tubes of the OVC, thermal radiation shield and cryogen vessel of a typical MRI system, have similar effective resonant frequencies when made from the usually-employed materials such as steel or aluminum.
The mechanical vibrations will be particularly strong when a resonant vibration frequency of a bore tube corresponds to the frequency of oscillation of the stray field. If the resonant frequencies of the OVC, thermal shield, cryogen vessel if any, and magnet components are close together, as is typically the case in present magnets, the bore tubes behave as a chain of closely coupled oscillators, and resonance bands will occur.
The oscillations may also interfere with the imaging process, causing detriment to the resulting images.
The resulting oscillations cause acoustic noise which is most unpleasant for a patient in the bore, as well as interfering with imaging and causing heating of cooled components such as the thermal radiation shield and cryogen vessel, if any.
The eddy currents induced in the cryogenically cooled components of the magnet constitute an ohmic heat load on the cryogenic cooling system, leading to an increased consumption of liquid cryogen where used, or an increased heat load on the cryogenic refrigerator. In dry magnets—those which are not cooled by a liquid cryogen—the increased heat load can result in a temperature rise of the coils, which can result in a quench.
Known approaches to this problem include the following. The gradient coil assembly may be mounted to the OVC bore tube 14b using resilient mounts, wedges or air bags. These are intended to attenuate the mechanical oscillations of the gradient coil assembly. However, such arrangements do not completely prevent mechanical transmission of vibrations from the gradient coil to the OVC, and do nothing to reduce the incidence of eddy currents in adjacent electrically conductive structures. It has been suggested to mount the gradient coil on to end frames, rather than to the OVC bore tube. However, such arrangements have required a lengthening of the system, which some embodiments of the present invention seek to avoid. Mechanical stiffening of the gradient coil assembly has been attempted. However, it is believed that a doubling of the stiffness of the gradient coil assembly will only result in an approximately 1.4× increase in the resonant frequency. Active force feedback actuators are suggested in U.S. Pat. No. 6,552,543, where actuators are placed within the OVC to oppose vibrations caused by stray fields from gradient coils. This solution is considered complex, and difficult to position the actuators between other components such as the magnet coils. It would be difficult to correctly synchronize the force feedback actuators with the oscillations induced by the gradient coils. Mode-compensated gradient coils have been suggested, in which primary and secondary conductors of the gradient coil assembly itself are optimized to reduce the amplitude of vibration of the gradient coil assembly. However, such optimization has been found to increase the stray field of the gradient coil assembly, resulting in increased heating of cryogenically cooled components due to eddy current generation.
Known approaches to similar problems have been described in the following publications.
U.S. Pat. No. 6,552,543 B1 discloses the use of mountings, including active mounts, between the gradient coil assembly and the cryostat.
U.S. Pat. No. 5,345,177 B2 this discloses the use of radial-spoke gradient coil mountings incorporating soft pads.
U.S. Pat. No. 6,353,319 B1 discloses mounting the gradient coil in the magnet bore, at points of maximum amplitude of mechanical vibrations, to disrupt resonant modes.
U.S. Pat. No. 7,053,744 B2 discloses a vacuum enclosure for the gradient coil.
U.S. Pat. No. 5,617,026 discloses the use of Piezo-transducers as a means of reducing the amplitude of gradient vibrations.
DE 10 2007 025 096 A1 discloses a method of mode-compensation of a gradient coil.
U.S. Pat. No. 6,954,068 teaches placing the gradient coil within an evacuated, electrically non-conductive vessel to reduce noise and vibration.